According to prisonlegalnews.org, "One in four private prison guards makes less than $26,091 annually," and further, "…employees of private prison companies earned roughly $6,000 to $9,000 less than their public counterparts." What this essentially means is that private sector incarceration facilities pay their prison guards less money and are provided with less material benefits than comparably based public incarceration institutions, which quite obviously is one of those factors that permits for-profit incarceration facilities to achieve a higher gross margin and to turn the necessary profit.
The fundamental thing about prisons is that the people that are incarcerated there are afforded "free" room and board, but are imprisoned, and therefore are subject to the vicissitudes of prison discipline and prison laws, both written and unwritten. So that, while very few people envy those that are imprisoned, the fact that those imprisoned do have food, water, exercise and entertainment facilities, library and/or education facilities, and a place to sleep, can be a source of displeasure for those that are their guards, especially when those prison guards perceive that the money that they earn in their work, barely allows them to keep their heads above water, and may not be really enough to keep them out of crushing debt, themselves.
This thus means that in any situation in which one party has sway and supervision over another party, in which those supervisors are underpaid, underappreciated, and are vulnerable to some real degree of physical harm and danger themselves; that they will be sorely tempted to look or to consider seriously augmenting their income so as to make their life appreciably better, especially when they believe, rightly or wrongly, that they deserve to have that extra benefit.
So then, while prisoners often lack ready cash, those that are on the outside, that are their family, friends, and associates, do have access to ready cash or its equivalency, in which the cost of product going into the prison, is always going to have a very hefty markup. Since, for the most part, nobody watches the watchers, but instead prisons are setup to watch the inmates as well as visitors of those inmates, the easiest and most common way to get contraband into a prison and to effectively ignore such contraband being there, is to utilize the best tool to do so, which are those underpaid and underappreciated prison guards.
The result is pretty intuitive, in which, those that are paid too little but are working a legitimate "square" job, in which they are struggling to live a life of any sort of quality, will find themselves understanding that since they have the authority over their captive audience, this thus allows them to make extra money, of which, those incarcerated, already understand the nature of the game, so that, they don't mind the price of doing business; so that, each side benefits from it. So too, the private enterprise incarceration complex, really doesn't mind either, because rather than having to pay a living wage to their employees, they will instead allow that short gap of wages to be paid instead by those that are incarcerated, in one form or another.
This signifies that the contraband that comes into prison facilities, primarily comes in via corrupt prison guards, of which, nobody will readily admit to such, but those that are part of the business, know such is true.